Why does one need to LEAN OUT a CARB when climbing?

Couple followups to the responses on this thread.

First, I'm not doubting that you have to lean at higher altitudes; that
is empirically most true. I'll try to restate the question below...
Stealth Pilot wrote:

"why do I lean my carb when I climb" is a bull**** question open to
much misinterpretation.

"why do I need to lean my carburettor at higher altitudes" is probably
a better wording of the question.

Sure, that's what I was asking in the original post.

that is simple. the air is less dense so the amount of fuel it needs
to achieve full combustion is reduced. the density of the fuel doesnt
decrease so you need less of it.

That I understand. It's also effectively a handwaving explanation to
the original question.

Here's the original question restated. Can somebody explain, why at
higher altitudes, the less dense air does not automatically meter less
fuel through a carb venturi, automatically making up for this
difference. A carb venturi operates taking advantage of the decrease in
pressure associated with the increase of velocity through the venturi --
a phenomenon which is dependent on the density of the air.

This question doesn't apply to a fuel injected engine, where as I
understand the fuel metering volume does not depend on air density, or
to any carbs with a compensating device.

wrote:
On Jan 19, 7:05 pm, "Todd W. Deckard" wrote:
It diverts low pressure air from the back of the venturi into the
fuel float bowl.
Never taken a close look at an airplane carb, but I understand that the
standard config (not this stromberg, but rather something like in a
basic 172N) has a fuel-bowl vent upstream of the venturi, making the air
pressure in the fuel bowl pretty much equal to the air intake
pressure.... Kind of like a standard auto carb from a few years ago when
they still had them. Let me know if that's not right.

On Tue, 20 Jan 2009 05:44:01 -0500, Tman
wrote:

Couple followups to the responses on this thread.

First, I'm not doubting that you have to lean at higher altitudes; that
is empirically most true. I'll try to restate the question below...
Stealth Pilot wrote:

"why do I lean my carb when I climb" is a bull**** question open to
much misinterpretation.

"why do I need to lean my carburettor at higher altitudes" is probably
a better wording of the question.

Sure, that's what I was asking in the original post.

that is simple. the air is less dense so the amount of fuel it needs
to achieve full combustion is reduced. the density of the fuel doesnt
decrease so you need less of it.

That I understand. It's also effectively a handwaving explanation to
the original question.

Here's the original question restated. Can somebody explain, why at
higher altitudes, the less dense air does not automatically meter less
fuel through a carb venturi, automatically making up for this
difference. A carb venturi operates taking advantage of the decrease in
pressure associated with the increase of velocity through the venturi --
a phenomenon which is dependent on the density of the air.


I think what actually happens in an aircraft carby is much cruder than
the elegant descriptions would have you believe.
in the marvel schebler ma3-spav there are a number of jets that all
need to be tweaked just so for the damn thing to work.

there is an accelerator jet which is pumped when the throttle goes in.
it also unpumps when the throttle gets pulled out but that just
refills it's barrel. the arm to this has three settings increasing or
decreasing the pump stroke and once you have this just sweet for your
engine dont change it. the outlet pipe needs to point just so into the
inner venturi ring.

there are a number of idle jets (actually just holes in the side) in
the wall of the carby that keep it running around idle when the
butterfly is almost closed. once this is adjusted for steady idle dont
change it.

then in the main jet comes in a whole number of sizes both in position
and size of the apertures that allow fuel to escape. there is also an
insert restricter in the base which is tweaked for each particular
engine type. dicking around with this to get a sweet running engine
can take ages.

as for the changes in velocity or density actually changing the fuel
flow well hmmmmm. if an engine can be run from sea level to about
6,000ft at full rich and still run and produce power, well that doesnt
seem like too much of a flow change with density.

jabiru use a bing altitude compensating carby. this has some sort of
altitude sensing compensation mechanism in addition to the usual
metering but that is a different animal.

the old style carbys have a lot of tinkering behind the steady smooth
metering in usual operation and have a simple adjustment that pilots
can use to keep the mixture in the runnable range at other times.

so my explanation is that if your surmise is correct I think it has
only about a tenth of the effect you think it does.
lots of tinkering and tweaking go behind making you think it does.

Stealth Pilot

On Jan 20, 6:32 am, Stealth Pilot
wrote:

as for the changes in velocity or density actually changing the fuel
flow well hmmmmm. if an engine can be run from sea level to about
6,000ft at full rich and still run and produce power, well that doesnt
seem like too much of a flow change with density.

Combustible mixtures range from 8:1 (air:fuel by weight) to
18:1. At sea level at full rich it'll be around 10:1. Pretty rich. As
you climb it'll head for 8:1 as the air's density decreases and the
engine will start to run rough and make black smoke. The mixture is
changing and you need to do something about it. We have airplanes here
at 3000' ASL that won't run nicely at all at full rich on a standard
day and we have to lean them for takeoff if it's any warmer than that.

Dan

Tman,

I've been pondering your question quite a bit. I believe I have it. In
deference to Dan my Camaro never idled correctly.

Ultimately your engine depends on the mass flow of air divided by the mass
flow of fuel. But the amount of fuel drawn up is a function of the pressure
difference in the carbureture venturi. So here goes:

The carb throat is a double venturi and a manometer between the opening and
the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
discouragingly this has the density
in front of it, which is why you posed the question. The difference in
pressures is directly proportional to the density.

Now the low pressure in the neck of the venturi is what is drawing the fuel
up (or properly the difference between the neck and ambient). Again we use
Bernoulli to describe the forces acting on a particle moving along a
streamline -- and this time it is properly incompressible.

{ Pressure / density } + .5 * { velocity ^ 2 } + gravity * change_in_height
= a constant

Again, Bernoulli copped it all from Newton and was just telling us that
kinetic energy + potential energy = a constant.

If we apply this to your fuel being drawn up we get:

(#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2 +
(gravity * vertical distance from bowl to jet)

However mass flow is the density * the size of the pipe * the velocity.

So the mass flow of air = density of air * carb barrel size * velocity
(opening)

But from (#) the mass flow of fuel is being determined by the pressure
difference (which also carries the air density)

so Air over Fuel cancels your density term.

An altitude compensating carburator puts a small vacuum on the fuel to
prevent the rho from dividing out.

Mr Wizard could have explained this better that as the air gets thinner it
sucks on the straw with less force but it takes less force to slurp up the
gas becuase of the reduced pressure. So the gas drawn up stays about the
same, however the mass flow of air drops off with density so the mixture
richens.

Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!

On Tue, 20 Jan 2009 08:30:34 -0800 (PST),
wrote:

On Jan 20, 6:32 am, Stealth Pilot
wrote:

as for the changes in velocity or density actually changing the fuel
flow well hmmmmm. if an engine can be run from sea level to about
6,000ft at full rich and still run and produce power, well that doesnt
seem like too much of a flow change with density.

Combustible mixtures range from 8:1 (air:fuel by weight) to
18:1. At sea level at full rich it'll be around 10:1. Pretty rich. As
you climb it'll head for 8:1 as the air's density decreases and the
engine will start to run rough and make black smoke. The mixture is
changing and you need to do something about it. We have airplanes here
at 3000' ASL that won't run nicely at all at full rich on a standard
day and we have to lean them for takeoff if it's any warmer than that.

Dan

thanks for the mix ranges dan. couldnt recall them.
do you have an answer for the guy's original question?

I think you are making this waaaay to complicated. The volume of air going
through the venturi remains the same as you climb but the amount of oxygen
(the component needed to burn the fuel) decreases. The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
There is less oxygen so the mixture become rich.

That's my story and I am sticking to it.

--

*H. Allen Smith*
WACO - We are all here, because we are not all there.
"Todd W. Deckard" wrote in message
...
Tman,

I've been pondering your question quite a bit. I believe I have it. In
deference to Dan my Camaro never idled correctly.

Ultimately your engine depends on the mass flow of air divided by the mass
flow of fuel. But the amount of fuel drawn up is a function of the
pressure difference in the carbureture venturi. So here goes:

The carb throat is a double venturi and a manometer between the opening
and the neck would show a theoretical pressure drop of:

p(opening) - p(neck) = .5 * density of air * { velocity(neck)^2 -
velocity(opening)^2 }
(Lets ignore carb ice for a second and say that the air is
incompressible).

Bernoulli got the idea from Newton thats why the 1/2 m v squared. Now
discouragingly this has the density
in front of it, which is why you posed the question. The difference in
pressures is directly proportional to the density.

Now the low pressure in the neck of the venturi is what is drawing the
fuel up (or properly the difference between the neck and ambient). Again
we use Bernoulli to describe the forces acting on a particle moving along
a streamline -- and this time it is properly incompressible.

{ Pressure / density } + .5 * { velocity ^ 2 } + gravity *
change_in_height = a constant

Again, Bernoulli copped it all from Newton and was just telling us that
kinetic energy + potential energy = a constant.

If we apply this to your fuel being drawn up we get:

(#) p (ambient) - p (neck) / density of fuel = .5 * (velocity of fuel) ^2
+ (gravity * vertical distance from bowl to jet)

However mass flow is the density * the size of the pipe * the velocity.

So the mass flow of air = density of air * carb barrel size * velocity
(opening)

But from (#) the mass flow of fuel is being determined by the pressure
difference (which also carries the air density)

so Air over Fuel cancels your density term.

An altitude compensating carburator puts a small vacuum on the fuel to
prevent the rho from dividing out.

Mr Wizard could have explained this better that as the air gets thinner it
sucks on the straw with less force but it takes less force to slurp up
the gas becuase of the reduced pressure. So the gas drawn up stays about
the same, however the mass flow of air drops off with density so the
mixture richens.

Q.E.D. Good question. If I ever become a physics teacher I am going to
put this one on the final!

On Jan 21, 7:19 am, "Allen" wrote:
I think you are making this waaaay to complicated. The volume of air going
through the venturi remains the same as you climb but the amount of oxygen
(the component needed to burn the fuel) decreases. The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
There is less oxygen so the mixture become rich.


More properly, the weight of the air decreases. Oxygen still
makes up 21% of the air.

Dan

On Jan 21, 1:16 am, Stealth Pilot
wrote:

thanks for the mix ranges dan. couldnt recall them.
do you have an answer for the guy's original question?

I think Todd Deckard answered that well. I'll leave the math
to those good with it.

Dan

Allen wrote:
I think you are making this waaaay to complicated.
I know I am. I'm an engineer, it's what we do...
Anyways.

The volume of air going
through the venturi remains the same as you climb
I believe this is a good approximation -- volumetric efficiency should
not change (much) as an aircraft climbs into thinner air, and throttle
setting and RPM's approx constant.

but the amount of oxygen
(the component needed to burn the fuel) decreases.
The mass flow decreases of air (and oxygen), and therefore the amount of
fuel required decreases; I agree.

The volume of air
remains the same so the fuel drawn out of the float bowl remains the same.
This is the part I don't agree with. The mass rate of fuel drawn outta
the float bowl is driven by the pressure decrease in the venturi, which
depends on both the density and the velocity. Density goes down as you
climb. Amount of fuel drawn should not remain a constant.

If I can give a rough and poor analogy. As you climb into less dense
air, you don't achieve the same aerodynamic performance from the
airplane with a given airspeed. You will stall at a higher airspeed,
your airplanes gear and flaps can handle higher airspeeds, Vx and Vy are
higher airspeeds, and Vne is a higher airspeed. The air has to go
faster to have the same effect on the wings and other devices on the
bird. It is because of the decreased density... I'd propose that is the
same effect as is happening in the carb venturi. The air has to go
faster to have the same effect in pulling gas out.

Oh yeah, before somebody corrects me I'm particularly talking about TAS
above, I know these speeds stay roughly the same at IAS (just because
the ASI is also thus affected ).


That's my story and I am sticking to it.
And most people are satisfied with that, I'm wondering why it doesn't
stand up to a little deeper analysis.